Good point on :serial t, but I don't tend to use it mostly because I'm afraid to work out the dependencies after the fact.
I use the REPL an awful lot. I use *, **, and *** a lot. I use the history a lot. I don't use slime-scratch buffers very much.
I'd be interested in seeing your mode of scratch form development in motion. (When I first saw slime screenshots, I thought it was no big deal, compared to ILISP. When I saw it in action, actually doing stuff, I was blown away...)